1. Field of the Invention
This invention relates to a process for electrographic image production and to an apparatus for carrying out this process. More particularly, the invention is concerned with the problem of cleaning a repeatedly usable intermediate image carrier of an electrographic copier. In the context of the invention, an "intermediate image carrier" is either a charge image carrier on which a latent electrostatic charge image can be produced, or a conductivity image carrier on or in which a latent conductivity image can be produced.
2. Description of the Prior Art
Various processes for electrographic image production are already known. The present invention relates in particular to processes of the kind in which a latent electrostatic charge image is produced on a repeatedly usable image carrier, more especially a charge image carrier comprising a photoconductor, the latent charge image being dry-developed by means of a so-called magnetic brush. The manner in which the electrostatic charge image is produced is of no significance to the present invention. Accordingly, the invention is applicable both to image-producing processes of the type in which a uniformly distributed electrical charge is applied in darkness to a repeatedly usable photoconductor as charge image carrier and the photoconductor thus charged is selectively discharged by exposure to light in dependence upon the original or master in order to produce an electrostatic latent charge image dependent upon the original, and also to so-called PIP image production processes where an internal polarization effect of the charge image carrier is utilized (cf. for example Journal of Applied Optics, 1969, Supplement 3, Electrography, pages 170-175, "Electrophotography by Persistent Polarization Due to Inverse Field", and also to other electrographic image production processes in which a charge image or a conductivity image has to be developed on a carrier).
In conventional magnetic brushes of the kind normally used for developing the above-mentioned latent electrostatic charge images, a toner powder, chargeable by frictional electricity, is used in admixture with magnetizable, powder-form material, for example, iron or iron oxide, as developer in order to produce a brush-like coating consisting of toner particles and magnetizable particles on a support equipped with at least one magnetic system, for example, in the form of an aluminum cylinder. Accordingly, the coating consists of two different components, namely the toner particles and the magnetizable particles. Accordingly, developers of this kind are normally referred to as two-component toners. The toner particles present in the coating are charged by frictional electricity, for example, with positive polarity. During the passage of the intermediate image carrier bearing a charge image, for example, with negative polarity, together with the coating of the magnetic brush, toner particles which previously have been positively charged by frictional electricity are removed from the coating by the charge image and retained on the intermediate image carrier corresponding to the latent charge image. A toner image is formed on the photoconductor or intermediate image carrier, being subsequently transferred to a second, definitive image carrier in known manner by a toner transfer process.
It has now been found that, following transfer of the toner image to the definitive image carrier, residues of the toner material are always left behind on, and soil, the intermediate image carrier. Conventional toner transfer processes have an efficiency of only about 60 to 80%, so that a significant proportion of the toner material previously transferred to the intermediate image carrier is left behind on it after the image has been transferred to the definitive image carrier.
Accordingly, the intermediate image carrier has to be cleaned, generally after each image-producing cycle but at the very least after a change of original, in other words toner residues left behind on it have to be removed in order to be able to commence the following image-producing cycle with a clean intermediate image carrier.
Cleaning may be carried out with brushes and, in particular, even with magnetic brushes of the kind referred to earlier on. Removal of the toner residues is improved by applying to the cleaning brush a voltage of opposite polarity to the toner residues left on the charge image.
It has also been proposed (cf. U.S. Pat. No. 3,647,293) to provide the image-producing apparatus with only one magnetic brush and to use this brush both for developing the latent image on the intermediate image carrier and also for subsequently cleaning the intermediate image carrier, the polarity of a d.c. voltage applied between the intermediate image carrier and the magnetic brush for the development process being opposite to the polarity of the d.c. voltage applied for the cleaning process.
One disadvantage of the above-mentioned conventional processes and apparatus is the considerable outlay involved in generating the above-mentioned d.c. voltage to be applied with alternating polarity, and in particular the changes in toner content and, hence, in the mixing ratio of the developer which occur in conventional processes and apparatus. The removal of toner particles for image production weakens the developer mixture, consisting of magnetizable material and toner particles, with increasing number of image-producing cycles. Accordingly, fresh toner particles have to be introduced at least periodically into the developer mixture in order to obtain images of uniform quality. Considerable difficulties are involved in obtaining the correct dosage for the fresh toner particles to be introduced and also in correctly mixing them with the developer still present consisting of magnetizable material and toner particles. The reason why these difficulties are so great is that it is extremely difficult in practice to determine the mixing ratio at any particular moment and, hence, to calculate the effective quantity of fresh toner required with sufficient accuracy. In addition, experience has shown that the magnetizable material also deteriorates during use of the developer, so that it also has to be periodically replaced. However, replacement of the magnetizable material involves considerable difficulties in practice on account of the very strong magnetic system present in the magnetic brush.